1. Field of the Invention
This invention relates to an oxygenator such as an artificial lung utilizing porous hollow fibers.
More particularly it concerns an artificial lung suitable for cases where the extracorporeal circulation periods extend over a long period of time and where an extended auxiliary circulation is required for patients suffering from respiratory failure.
2. Description of the Prior Art
The prior oxygenators may be classified roughly into the bubble type and the membrane type. Recently the latter is recommended for use because it damages the blood to a lesser extent. The membrane type oxygenator supplies oxygen to one side of a sheet of silicone membrane of the oxygen diffusion type, and the blood to the other side thereof, thereby causing an oxygen and carbon dioxide exchange through said membrane. However, this type of artificial lung was defective in that the use of a flat membrane necessarily increased the size of the apparatus, that the silicone rubber membrane is easily damaged as it contacts the membrane support member, and that extreme care is needed in handling the apparatus because of its extreme instability.
In view of these defects, the inventor of the present invention proposed a hollow fiber type artificial lung (disclosed in U.S. Pat. No. 4,239,729). The artificial lung disclosed in U.S. Pat. No. 4,239,729 has a construction wherein a bundle of hollow fibers consisting of a microporous membrane having excellent gas permeability is disposed instead of a silicone rubber membrane inside a housing, and the bundle is fastened at its center portion by an inward projection provided within the said housing. This artificial lung supplies the blood inside the hollow fibers, and oxygen into the space defined by the housing and the outer surface of the hollow fibers, thereby facilitating gas exchange of oxygen and carbon dioxide through the microporous hollow fibers. According to this artificial lung, various merits are achieved such as a higher gas exchange efficiency, excellent sturdiness of the apparatus, and compactness of the apparatus by the use of the microporous membrane with an improved gas exchange property. Uniform dispersion of oxygen gas and channeling prevention is achieved by provision of the inward projection which acts to fasten the hollow fiber bundles.
As a result of an experiment using the above mentioned oxygenator for an extended period of time, the inventor found out that the gas exchange efficiency deteriorated gradually as the water accumulated at the point where the inward projection fastens the hollow fiber bundle inside the oxygenator. In other words, the microporous membrane allows the steam to pass therethrough while it has an excellent gas exchange property so that the steam becomes condensed and forms dew as it contacts the inner wall of the housing after passing through the hollow fibers. When in use, the oxygenator is usually placed with its axial direction in respect of the earth so as to facilitate debubbling at the time of priming and discharging the water that accumulated. The water content which forms dew beneath the inward projection flows down to the lower part of the apparatus and causes no problem. However, the dew formed in the upper section of the apparatus accumulates in a pool above the inward projection. Accordingly, after the oxygenator has been used for an extended period of time, the water pool prevents the excessive passage of oxygen and then lowers the gas exchange efficiency.